Wrenching in the external zone of the Betic Cordilleras, southern Spain

Tectonophysics, 107 (1984) 57-79 57 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

W R E N C H I N G I N T H E E X T E R N A L Z O N E O F T H E B E T I C C O R D I L L E R A S ,

S O U T H E R N S P A I N

M.E.M. DE SMET

Institute of Earth Sciences, Free University, P.O. Box 7161, Amsterdam (The Netherlands)

(Received September 2, 1983; revised version accepted January 16, 1984)

ABSTRACT

ABSTRACT

De Smet, M.E.M., 1984. Wrenching in the external zone of the Betic Cordilleras, southern Spain. Tectonophysics, 107: 57-79.

Regional structural trends in the Caravaca-Huescar area, forming the central part of the External Zone in the Betic Cordilleras, indicate that deformation of the area results from right lateral wrenching, subparallel to the paleogeographic zonation. The structural trends do not fit the nappe interpretation generally used elsewhere in the External Zone.

Arguments for wrenching in the area are: the structural inverse symmetry with respect to a central transcurrent fault zone (the Crevillente Fault Zone), the distributional pattern of stratigraphic anomalies and types of deformation, and the outcrop pattern of the paleogeographic zones.

It is argued that certain tectonic units in the area, formerly interpreted as klippes in a nappe configuration, form centers of "flower structures" and are vertically squeezed out blocks.

A theoretical model is sketched, illustrating the evolution of a wrench zone under the stratigraphic conditions of the studied area and resulting in its present characteristics.

Many of the features of the Caravaca-Huescar area are also present elsewhere in the External Zone and support the idea that wrenching was a major deformational mechanism. It is therefore suggested that the External Zone should in general not be described in terms of a nappe structure, ~s has been done so far, but in terms of a strike-slip orogen or a wrenched continental margin.

INTRODUCTION

T h e Be t i c C o r d i l l e r a s a re t he A l p i n e m o u n t a i n be l t o f s o u t h e r n Spa in . G e o l o g i -

ca l ly it is s u b d i v i d e d i n t o the I n t e r n a l Z o n e a n d E x t e r n a l Z o n e (F ig . 1).

T h e I n t e r n a l Z o n e m a i n l y c o n s i s t s o f p a r t i a l l y m e t a m o r p h o s e d P r e c a m b r i a n to

P e r m o - T r i a s s i c rocks . I t f o r m s p a r t of t he A l b o r a n p l a t e ( A n d r i e u x et al., 1971,

1973) , a l so k n o w n as K a b y l o - B e t i c o - R i f i a n m a s s i f ( P a q u e t , 1974). T h e L a t e M e s o -

zo ic p o s i t i o n o f th i s p l a t e is g e n e r a l l y a s s u m e d to h a v e b e e n r e l a t i v e l y f a r t h e r eas t in

t h e M e d i t e r r a n e a n a r e a ( A n d r i e u x a n d M a t t a u e r , 1973; P a q u e t , 1974; B o u r r o u i l h

a n d G o r s l i n e , 1979; Bourgo i s , 1980).

0040-1951/84/$03.00 © 1984 Elsevier Science Publishers B.V.

58

4 2 *

4 0 °

1 0 ° 15 ° 6 °

r

J

1 ;

s b o n "

3 8 * !

,~TLANTIC 3 6 * _

OCEAN

I I M a d r i d

0 * 2 a

P Y R E N E E S !,

4 ~

IMESETA

, A L B O R A N

r S E A

' t e

i i

~ A e n o r c a

M a l l o r c o

Fig. 1. Locat ion of the Betic Cordilleras. The mounta in range is geologically subdivided into an Internal

and External Zone (C.H.A. = Caravaca-Huescar area).

The External Zone consists of unmetamorphosed Mesozoic and Tertiary deposits of the continental margin that evolved along the southern side of the Iberian plate as a consequence of the break up of the Pangean continent (Hermes, 1978a; Garcia- Hernandez et al., 1980).

Late Mesozoic and Tertiary movements of the Iberian and Alboran plates finally caused their collision. At Burdigalian times a contact had come about between External and Internal zones (Hermes and Kuhry, 1970; Durand Delga, 1980). Plate movements resulted from the Eocene onwards but especially during the Early Miocene in a strong deformation of the External Zone. It is the character of this deformation process which forms the subject of the present paper.

Classical interpretation of the complex structure of the External Zone has been mainly within the frame of a nappe model. Different authors have proposed some different ways of development of the nappe structure but the overall mechanism is generally supposed to have been N N W directed overthrusting, caused by compression perpendicular to the orogens axis (Blumenthal, 1927; Staub, 1934; Fallot, 1948; Durand Delga, 1966; Garcia-Hernandez et al., 1980).

59

Seyfried (1978) criticized the model in a paper on Jurassic stratigraphy in part of the External Zone. He concludes that a nappe structure cannot be substantiated.

Nappe interpretation was applied to the centrally situated Caravaca-Huescar area (Fig. 1), but in this area it appeared inappropriate to result in a structural synthesis. On the contrary, discrepancies with the model were discovered and strike-slip faulting appeared to have played a far more importan t role than was previously assumed. Two dextral transcurrent faults, subparallel to the axis of the orogen, were described by Paquet (1969, 1972). Hermes (1978a) suggested the existence of an interbranching system of such faults in the External Zone and interpreted some major features of the Caravaca-Huescar area as the result of large scale movements along it. Van de Fliert et al. (1980) gave attention to the stratigraphic anomalies caused by the transcurrent faulting. Further research in the area (De Smet, 1984) revealed that its entire structure reflects one big dextral wrench process subparallel to the paleogeographic zonation. Characteristics of the structure and arguments for the wrench hypothesis are discussed below after an introduction to the geology of the External Zone.

GEOLOGIC SETTING OF THE EXTERNAL ZONE

Figure 2 illustrates the authors interpretation of the build up of the continental margin along the Iberian plate shortly before its main deformation began in about

o o i o 4 ° E x t e r n o l I n t e r n a l E x t e r n o l Med ion I n t e r n a l

Units

P r e b e t i c S u b b e t i c

: : ~ Car'bonat e p la t fo rm deposi ts

I ~ Sands

~ Pelagic sed imen ts

~ - ~ I nt r-u s i res / Pi l low lava' . ~

~ Evopor-ite$

r ? T Turb id i tes

~ S t P a t i g r a p h i c gop

/ \ "V::.cRust

1"1 Fig. 2. Hypothetical reconstruction of the Mesozoic and Tertiary continental margin bordering the Spanish Meseta. Deformation of the margin, in particular during the Early Miocene, resulted in the present External Zone of the Betic Cordilleras. Six paleogeographic subzones are distinguished. Paleogeo- graphic relation of the Internal Subbetic (subzone 6) to the other subzones is not well known.

60

Early Miocene times. Six paleogeographic subzones are shown, corresponding with

the six main subzones generally distinguished in the present geologic setting of the

External Zone (Fig. 3). The paleogeographic relation of subzone 6 (Internal Sub-

betic) to the other subzones is not well known and is subject to discussion. A z e m a et

al. (1979) and Garcia-Hernandez et al. (1980) assume that it was deposited directly

south of subzone 5 (Median Subbetic), whereas Hermes (1978a) presents arguments

to consider it as a paleogeographically separate unit. This discussion has, however,

no direct impact on the subject of this paper, since all authors agree that a contact

between subzones 5 and 6 existed at the beginning of the Miocene. Various aspects of the External Zone are illustrated in Fig. 3. Figure 3A shows the

outcrops of sediments from the different paleogeographic subzones and separately

I B E R I A N

• + + + + + ~ . J

+ + + + ~ + + + ~f •

::::::::::::::::::::: \\

c~

f o o

Go~o ~°°~ f - ~ ........

/ /

/ /

' i i :

CREVlLLENTE -

i J .......... _ LEGEND

EXTERNAL ZONE

T r i a s s i c r o c k s

AM~P t ~ ( ~ ) l n * , . . . . mate Umts

' / t t ® E x ' . . . . ' ' ° ° ° ° " '

] L ~ j ~ ) Median S u b b e t c

{ ~ ) I . t . . . . [ S u b b e t , c

Fig. 3. Simplified geologic setting of the External Zone (Map A). Triassic rocks are shown apart from younger rocks in which the paleogeographic subzones are distinguished. The framed area is the Caravaca-Huescar area, shown in Fig. 4. Map B gives a survey of the distributional areas of rocks from the paleogeographic subzones. Data are mainly after Lopez-Garrido and Vera, in Azema et al. (1979).

61

the outcrops of Triassic rocks. These latter rocks form an important constituent of the External Zone and even dominate the geological picture on many places. They consist of evaporites and other shallow water deposits which originated before the continental margin developed. Therefore, they fall outside the distinction between the paleogeographic subzones. Figure 3B is a schematic map of the areal distribution of outcrops of the six subzones. The map demonstrates that the distribution has remained rather simple, despite the deformation to which the External Zone was subjected. The original sequence of the subzones (1-6) has not changed fundamen- tally with regard to the Spanish Meseta.

Another important feature is the difference in structural style between the outcrop areas of subzones 1 and 2 on the one hand (Prebetic), and those of subzones 3-6 on the other (Intermediate Units and Subbetic). This difference can be related to a difference in rheologic characteristics. Subzones 1 and 2 (shelf area) mainly consist of rigid limestones and sandstones with a thick development on the distal side. At present they form a structurally coherent area with relatively large scale structures. Subzones 3-6 on the other hand (slope and rise area, at least from the Jurassic onwards) are predominantly composed of marls and sequences of alternat- ing marls and limestones. Their present outcrop area shows an intensely faulted type of geology in which large quantities of mobile Triassic rocks separate blocks of younger sediments (Jurassic-Tertiary) (Fig. 3A). Additional features are a great variety of types and orientation of structures.

Finally the Crevillente Fault Zone must be mentioned. It has a length of several hundreds of kilometers and runs subparallel to the axis of the External Zone, truncating and separating the outcrop areas of the paleogeographic subzones (Fig. 3). The giant fault zone runs centrally through the Caravaca-Huescar area and plays a crucial role in the wrench hypothesis.

There is an extensive body of literature on the geology of the External Zone, but since there are still many local problems only few authors try to give structural interpretations for the entire belt. Stratigraphic reviews and paleogeographic reconstructions are given by Hermes (1978a) and Garcia-Hernandez et al. (1980).

THE CARAVACA-HUESCAR AREA

For finding keys to the large scale structure of the External Zone, the Caravaca-Huescar area seems especially suited. The reason is that only in the central part of the zone, where this area is situated, all six paleogeographic subzones are contiguously exposed from north to south (Fig. 3). In the areas more to the west large parts of the proximal subzones are missing, while in the east the distal ones are absent.

De ta i l ed stratigraphic and structural data from large parts of the Caravaca-Huescar area are presented by Paquet (1969), Foucault (1971) and Hermes (1978a). A number of Jurassic sequences are described by Seyfried (1978).

62

2 "~ ' 2 ,20 r 2 ' 10 2 ' 00 ' 1 "~ ' 1,40 ,

s t r a t con tac t

tec ton ic contac t

s t r ike-s l ip faul t

t h rus t faul t

, , , , no rma l fau l t

~ - - ant ic l ine

o v e r t u r n e d anti¢l.

syncl ine

--JEt--- o v e r t u r n e d syncl. o l o 20kin

7 7 P o s t - o r o g e n i c depos i ts

i Triassic (main ly gypsum)

P R E B E T I C

01 T e r t i a r y

u ~ Cretaceous Z ~I ~ Jurass ic r~ w SUBBETIC I ' - LO × ~ Tertiary & Cretaceous

(strongly tectonized end mixed) ~ Jurassic

b,ed on dlta f;om: J J.HERMES (197kj, ,I.R.VAN OE FLIERT I t II.(1HO).A.FOLICAULT (1971). J P~QUET (1H91. P.J+HOEOIEMAECI(ER

(1973). T,GEEL (I|73], fl.SQEDIQNO (11171), 6 W.VAN VEEN) (I UD), I.G.M,E,ItOlqil ~ps

(~:50~1(I) of ~e |r~; end f u rdw~ | . n~lllidNd rop~rll by $.BAL. 6.J.A.BRUMMER B.ENgBRENGHOF. E.Q ERVAIS. H.GflAVEN.

0 P HANSEN. M.HULSI01. W.H I~0 It. H JANSEN. 0 KROON, LP.SMIT. AJ.IPE E LMAN,

R S.C.DE RUITE R

~ ROCK UNITS OF THE VELEZ RUISIO CORRIDOR (undiff.)

-'[--~ INTERNAL ZONE (undiff.)

Fig. 4. Geological sketch map of the Caravaca-Huescar area. A distinction is made between a Prebetic

and a Subbetic province. Note the difference in structural style of these two provinces.

63

Geologic maps from the Spanish Geological Survey are available (IGME-maps, 1 : 50,000, nos. 908-911,929-932, 951-953, 973-974).

Figure 4 is a schematic geologic map of the area, compiled by the author. A distinction is made between two geologic provinces: the Prebetic and the Subbetic. The Prebetic is composed almost exclusively of outcrops of deposits from subzones 1 and 2. It forms the geologically coherent area in the north of the map with a regular structural pattern. The Subbetic province, including the Intermediate Units, occupies the major part of the map and consists of outcrops of rocks from the subzones 3-6. In contrast to the Prebetic, the Subbetic is strongly faulted and folded. It shows an irregular mosaic of tectonic units, which in reality is far more complicated than could be represented on the scale of Fig. 4.

The Crevillente Fault Zone is recognizable as a central WSW-ENE running belt, in which numerous small tectonic blocks occur dispersed in a matrix of Triassic rocks.

In the following paragraphs main features of the Caravaca-Huescar area and in particular those of the Subbetic province are described.

THE SUBBETIC DEFORMATIONAL PICTURE

Deformation of the Subbetic has been very intensive. The province is composed of many loose fragments of rock sequences, often covering small parts of the stratigraphic column only. On many places rock units of different ages occur in a rather random tectonic arrangement, independent of original stratigraphic relations. In this situation a marked differential deformation exists of the three major rheological units constituting the Subbetic stratigraphic column: the Triassic, the Jurassic and the Cretaceous-Tertiary. Roughly speaking, Triassic rocks consist of gypsum and marls with intercalations of sandstones and dolomites. These predominantly show plastic deformation. Jurassic rocks consist of dolomites and pelagic limestones, characterized by a relatively rigid behaviour. Cretaceous and Tertiary sediments are composed of marls with intercalations of turbidites and pelagic limestones, which again deform plastically for a large part. The three units are often separated by tectonic boundaries (Fig. 4). Furthermore, they not only show different modes but also rather consistently different rates of deformation: the rigid Jurassic carbonates form large tectonic blocks with large scale folding, while Triassic, Cretaceous and Tertiary sediments are generally intensely folded and faulted and often tectonically mixed. Apparently deformation in the Subbetic has been such intense that rigid stratigraphic units hardly anywhere could protect overlying or underlying rocks against major disarrangements, detachment and mixing.

STRATIGRAPHIC ANOMALIES

The intense Subbetic deformation involved major displacements of tectonic units. This appears from the numerous stratigraphic anomalies between the units. Time

~4

equivalent sequences of neighbouring blocks sometimes show striking differences in

combinations of formations, thicknesses, fossil content, intercalation of peculiar

deposits, etc. In a number of cases these anomalies can be interpreted mainly as the

result of rapid lateral facies changes and only small relative displacements of the

blocks. In other instances, however, sequences have such incompatible characteristics

that one can but speculate about their paleogeographic distance at tile time of

deposition. Structural arguments to estimate the size of relative movements are hard

to find, since Subbetic structure, even on a local scale, shows varying styles and

varying directions. Consequently it depends very much on the interpretational

framework of the observer as to what origin for the anomalies is suggested. Hermes (1978a), describing the frequent occurrence of stratigraphic anomalies in the south-

ern part of the Subbetic, suggests that this part is composed of tectonic blocks

brought into each others vicinity by transcurrent movements over hundreds of

kilometres. Foucault (1971), working in the northern part of the Subbetic, where

such anomalies are less frequent and structure is more consistent, assumes that facies

changes rather rapidly. Following his interpretation there is no need to suppose that

relative movements of tectonic units exceeded a few to some tens of kilometres.

A concentration of stratigraphic anomalies is found in the central fault zone of

the Subbetic: the Crevillente Fault Zone. In this zone many tectonic blocks have

their very own stratigraphic characteristics (Hermes, 1978a; Van de Fliert et al.,

1980). Stratigraphic descriptions of fault zone blocks in the eastern part of the

Caravaca-Huescar area are presented by Paquet (1969). Blocks in the central part

are described by De Smet (1984). All authors mentioned explain the concentration

of anomalies at least in part as the result of maior right-lateral strike slip move-

ments along the zone. Apart from the stratigraphic anomalies dispersed throughout the Subbetic, major

anomalies also occur along the contacts of the Subbetic with its neighbouring

northern and southern areas. The nature of these contacts will be discussed later.

PALEOGEOGRAPHIC ZONATION

Although Subbetic geology is characterized by a lateral inconsistency of structures

and stratigraphic successions, trends are visible on a regional scale. The stratigraphic

trend is that the paleogeographic zonation, as illustrated in Fig. 2 and 3, can still be

distinguished in its original order from north to south. This zonation is based on

broad sedimentological criteria, such as relative proximality of deposits and large

scale build up of sequences. Along the northern border of the Subbetic province

tectonic units occur with characteristics of the Intermediate Units (subzone 3); more to the south, but north of the Crevillente Fault Zone, tectonic units show the

characteristics of the External Subbetic (subzone 4). In the fault zone itself at least a number of stratigraphic successions can be correlated with the Median Subbetic (subzone 5) and in the area south of the fault zone they can be correlated with the

65

Internal Subbetic (subzone 6). Although no accurate boundaries between the subzones can be mapped because of local tectonic mixing and because interpretational

problems, the overall change of the stratigraphic pattern in the Subbetic is striking

from north to south. This change is especially clearly revealed by the stratigraphic differences of the areas on both sides of the fault zone (Hermes, 1978a; Van de Fliert et al., 1980). Across this zone no exchange of stratigraphic elements seems to have occurred.

The fact that a paleogeographic zonation can still be distinguished in its original order indicates that relative displacements of tectonic units were of restricted size in

a direction perpendicular to that zonation. It suggests therefore that the intense deformation and the stratigraphic anomalies in the Subbetic were mainly caused by

displacements subparallel to the paleogeographic zones, which is also subparallel to the Crevillente Fault Zone.

THE INVERSE SYMMETRIC STRUCTURE OF THE SUBBETIC

Structural trends in the Subbetic show an inverse symmetrical picture, with the axis of the Crevillente Fault Zone as its mirror plane (Fig. 5). The elements of the

symmetric relation are: the outer zones of outward thrusting, the areas of varying

deformation with an outward directed trend of vergences and third, the central zone of intensive deformation and subvertical faults:

The outer zones of outward thrusting. Almost everywhere the Subbetic is bordered by thrust zones. The general picture is one of Jurassic carbonate massifs overthrusting highly deformed younger rocks, which on their turn overthrust the adjacent areas. Along the N N W border of the Subbetic, lenses of Triassic gypsum are also incorporated in the zones. In the northwest these thrust zones are described by Foucault (1971) and by Garcia-Hernandez et al. (1973), in the north they are described by Hoedemaeker (1973) and Van Veen (1972), in the east by Paquet (1969,

1972) and in the south by Soediono (1971) and Geel (1973). Only in the south, west

of Fuensanta, is it uncertain whether the thrust zones overlie the neighbouring areas.

Estimates of the size of Subbetic overthrust differ (ranging from virtually zero to

about 20 km), but are relatively restricted when compared with the dimensions of the entire Subbetic.

Areas of varying deformation with an outward directed trend of vergences. The areas between the outer zones of outward thrusting and the central Crevillente Fault Zone exhibit very diverse styles of deformation. Folds and faults have a wide range of

types- and orientations, giving rise to interpretational problems when going from one location to an other. On a regional scale, however, vergence directions of folds and reversed faults show two trends (Fig. 5): most frequently structures are facing away from the Crevillente Fault Zone and towards the borders of the Subbetic, implying that rock units were mainly pushed away from the fault zone. The second trend is bound to the immediate surroundings of Jurassic carbonate massifs. In these areas

66

2°30 ' 2, 20. 2 * 10 2 ~ 00' 1" 50 1"40'

strat, contact

tectonic contact

strike-slip fault

thrust fault

. . . . normal fault

anticline

overturned ant ic l

- - ~ syncline

overturned syncl. 0 10

~ _ _ ~ ~ VELEZ RUBIO CORRIDOR Post orogenic deposits (fault zone)

~ - t P R E B E T ' C ~ INTERNAL ZONE

Outer zones of outward thrusting

Areas of varying deformat ion with an outward U directed trend of vergences

Central zone of very intensive deformation and m subvertical faults (tectonic blocks in a matr ix of gypsum)

co a I ~ Vergence trends of structures

b J ~ l ~ a: local b: regional

__ ---- Mirror plane of the inverse symmetrical structure

Fig. 5. Structural trend map of the Subbetic province in the Caravaca-Huescar area. The province shows

an inverse symmetric pattern with the axis of the Crevillente Fault Zone as its mirror plane. Explanation

see text.

67

vergences are often directed away from the massifs, independently of the first trend.

Apparently these rigid units had a special role in the deformation process of the area (see below: flower structures).

Central zone of intensive deformation and subuertical faults. The Crevillente Fault Zone forms the axial part of the inverse symmetric structure of the Subbetic. It is characterized by an extreme rate of deformation and by a predominance of subvertical faults. Tectonic blocks ranging in size from decimetres to kilometres float in a matrix of highly distorted Triassic gypsum. The structural style of this zone can be described as a tectonic mega-breccia. Rigid rocks form the larger components, well layered sediments occur as folded sheets, brittle rocks are disrupted into countless smaller lenses, whereas gypsum forms the matrix (De Smet, 1984). A similar breccia

can only be the result of long lived motions along the zone.

The inverse character of the symmetric relationship appears from the regional

vergence trends of the structures. North of the fault zone compressive deformation is relatively strong in a WNW direction. South of the fault zone it shows a ESE direction.

From the symmetric structure of the Subbetic it can be concluded that the

Crevillente Fault Zone has played a crucial role in the overall deformation process. The zone not only forms the geometric axis of the area, but also the centre, from which compressive deformation was directed away and along which large scale movements occurred.

FLOWER STRUCTURES

The tectonic position of the Jurassic carbonate massifs in the Subbetic is of special importance for the general structural interpretation. The massifs form the highest parts of the relief and in general they show on most sides an upthrusted or overthrusted position on strongly deformed surrounding rocks (Fig. 4). This position

seems suggestive of klippes in a nappe structure and formerly it was'considered as a

confirmation of the generally existing idea that the External Zone consists of a pile

of nappes (see introduction). A study of all such massifs in the Subbetic of the

Caravaca~Huescar area revealed, however, that many have structural features in common which make their general interpretation as klippes less likely (Fig. 6A). In

the first place, the massifs are not bordered by consistent detachment horizons. Bordering faults usually cut through stratification and only occasionally Triassic rocks or the base of the Jurassic succession outcrop along the faults. Secondly, the faults generally dip towards points far below the massifs, while most massifs internally reveal an anticlinal structure. In a nappe configuration this would mean

that the concave parts of the "overthrust plane" would contain the convex portions of the "nappes" . Thirdly, as mentioned before, there is the diverging trend of structures in the immediate surroundings of massifs, which suggests for example on the northern side of a massif that the "nappe" moved northwards and on the

68

A "Nappe interDretation"

T

B "Flower structure interpretation

~ - ~ I Cretaceous-Tertiary Jurassic Triassic

Fig. 6. Comparison of nappe interpretation (A) and flower structure interpretation (B) for the tectonic

position of the Jurassic carbonate massifs in the Subbetic province. Previously it was assumed that the

massifs were klippes in a nappe structure. In the present paper it is argued that they form nuclei of flower

structures.

southern side that it moved southwards. A final problem in interpreting the massifs as constituents of nappes is the almost complete lack of rootzones in the area.

The combined features can, however, be well explained with the concept of "flower structures" (Figs. 6B and 7). The term "flower structure" is used in seismic stratigraphy for indicating complex structures, showing an upwards diverging fan of reversed faults. Name and descriptions of such structures come from Harding and Lowell (1979), who relate them to wrench processes. Their development is enhanced where strike-slip faulting is accompanied by components of convergence and where rocks are highly mobile (Lowell, 1972; Harding and Lowell, 1979). In the concept proposed here, the flower-hke geometry of reversed faults is of course interpreted.

Assumed development of flower structures under simplified Subbetic stratigraphic conditions is reconstructed in Fig. 7. Convergent wrenching first leads to faulting and some folding in the competent Jurassic rocks. Subsequently, the higher, anticlinal parts are squeezed out of their competent surroundings of other Jurassic blocks and then start to bulldoze away and overthrust the overlying incompetent rocks into several directions.

This way of interpreting the origin of at least a number of the Jurassic carbonate

69

g

i Triossic

Fig. 7. Idealized reconstruction of flower structure evolution, caused by convel;gent wrenching in a Subbetic-like situation. Convergent wrenching first leads to faulting and some folding in the competent Jurassic rocks. Subsequently the higher, anticlinal parts are squeezed out of their competent Jurassic surroundings and start to bulldoze away and overthrust younger incompetent rocks into several directions.

massifs in the Subbet ic is suppor t ed by the s t ructural deve lopmen t of the Sierra de

Mar ia . This is the range of Jurassic massifs a long the southern bo rde r of the

Subbet ic , west of Velez Rub io (Fig. 4). The two massifs d i rect ly nor thwest of Velez

R u b i o have abou t the same character is t ics as o ther Jurassic massifs (Fig. 6A and B,

sect ions Z Z ' , southern side). When the range is fol lowed in a wester ly d i rec t ion we

see, however, that nor th of Chirivel the two massifs form only the ant ic l inal nuclei

of, f rom nor th to south, two succesive ant ic l inal structures. Cre taceous depos i t s have

a no rma l s t ra t igraphic pos i t ion in the centra l syncl inal nucleus. This consis tent fold

70

structure, although more closely folded than the one shown in Fig. 7, can be considered as the early stage of flower structure development. The situation northwest of Velez Rubio can be considered as the late stage. Structural details of the Sierra de Maria will be discussed by Van de Fliert et al. (in prep).

The flower structure concept has a key function in the large scale structural interpretation of the Subbetic, because as long as the Jurassic carbonate massifs are

interpreted as klippes, it remains necessary to assume that they come from some

undefined place inside or outside the Subbetic by means of large scale overthrusting.

For such overthrusts are no further indications. When they are, however, considered as blocks squeezed out of their surroundings, it perfectly fits the interpretation of the

Subbetic as part of a wrench zone. This is further illustrated in the following sections.

WRENCHING IN A SUBBETIC TYPE STRATIGRAPHIC SITUATION (THEORETIC MODEL)

Principles of wrench tectonics are reviewed by Wilcox et al. (1973). Development of an outward verging pattern of structures in a laboratory model of a wrench zone, has been recently described by Odonne and Vialon (1983). Extending the principles,

a theoretic model can be built up for wrenching in an area with the stratigraphic characteristics of the Subbetic (Fig. 8). The genetic sequence in this figure illustrates

a highly simplified situation. The "Subbetic" is represented as a central zone of weakness, wrenched between two more rigid areas (" Prebetic" and "Internal Zone").

The mode of deformation in the "Subbetic" is mainly determined by the rigid

Jurassic carbonates. These deform quite independently from the basement, because

of underlying plastic Triassic rocks, and control plastic deformation of overlying younger deposits. To emphasize the mode of deformation of the Jurassic carbonates, younger rocks have been omitted in Fig. 8.

Early stage (Fig. 8A): In an early stage of wrenching an en 6chelon system of structures originates in the carbonates. Characteristic for wrenching is the develop-

ment of two conjugate sets of strike-slip faults (Riedelshears and conjugate Riedelshears; Tchalenko and Ambraseys, 1970). A strong fragmentation is the result and separate tectonic blocks subsequently deform according to local circumstances.

Middle stage (Fig. 8B): A highly varying deformation develops in the wrench

zone. Some blocks are faulted again, while others are folded or rotated. Moreover, differential vertical movements of the blocks begin to play an important role. The blocks may override each other into several directions and some may rise completely above their surroundings (flower structures). A diapiric behaviour of the underlying Triassic gypsum could facilitate such vertical movements.

Another effect of the fragmentation of the rocks is that the strike-slip component of additional wrenching concentrates along faults with a favorable orientation i.e. subparallel to the wrench direction (Wilcox et al., 1973). These faults merge into an interbranching system, which in the course of the wrench process gradually narrows

71

A. ear ly stage

IIUJlL iLOILO]l

H]IN1

B. middle stoge

C. late stage

Fig. 8. Idealized reconstruction of wrench zone development in a Subbetic-like situation~ In an early stage (A) Jurassic carbonates break up into tectonic blocks. Subsequently (B) blocks start to be pushed away

from the axis of the zone, where mobile Triassic rocks (black) fill up originated space. In adjacent areas some blocks are squeezed out of their surroundings (flower structures), while thrustzones develop along the wrench zone borders (C).

into a central, continuous strike-slip zone. During this process Triassic gypsum may start behaving as a lubricant between the blocks of Jurassic carbonates.

Late stage (Fig. 8C): The developments during the middle stage become more

pronounced when wrenching continues. A greater diversity of local structures originates. However, the concentration of strike-slip processes along a central zone leads to a decrease of strike-slip motion along faults in other parts of the wrench zone. In those parts deformation gradually acquires the characteristics of drag along the central zone. Drag results in both compressional and tensional forces. In an incoherent geology compressional forces may propagate from block to block to the

72

PREBET I S U B B E T I C I N T E R N A L Z O N E

+ ÷ + ~ / / /

I+ , * , + , + , * ,÷ , -~ + ÷ \ l , . ~ l W - ~ / • ~ . . . .

Fig. 9. Block diagram illustrating the late stage of wrenching in a Subbetic-like situation corresponding to

Fig. 8C. See Fig. 7 for legend.

external areas of the wrench zone. Tensional forces on the other hand remain more concentrated near the weak, central strike-slip zone, where mobile rocks such as Triassic gypsum, fill up the newly formed space between blocks. This development

can be illustrated by the comparison with drift ice along the banks of a swift stream.

The drift ice is piled up and even pushed onto the banks, while in the main current

the water may become virtually ice free. In a similar way the rigid Jurassic blocks are pushed away from the central strike-slip zone to the outer parts of wrenchzone.

Consequently, the structures tend to verge away from the central zone and thrust-

zones arise the wrench zone borders. Meanwhile mobile Triassic rocks flow towards

the deformational axis of the zone and become concentrated there. The blockdiagram of Fig. 9 illustrates once more this late stage of wrenching in a

Subbetic type stratigraphy. Here, however, the post-Jurassic rocks are included.

THE CARAVACA-HUESCAR AREA AS PART OF A WRENCH ZONE

The structural characteristics of the Subbetic in the Caravaca-Huescar area correspond with those of the portrayed late stage of wrenching (cf. Figs. 5 and 9). Wrenching seems to provide a clue to the interpretation of both local and regional phenomena in the area. The distribution of stratigraphic anomalies is in accordance with the model. Furthermore, since wrenching was subparallel to the paleogeo-

graphic zonation, the order of the zonation could more or less remain intact. As suggested in the figures, the wrench process in the Subbetic was not simple

parallel but convergent. A compressional component, perpendicular to the paleogeographic zonation, was involved. This can not be deduced from the structure of the area, but must be supposed on basis of the fact that the sedimentary elements of a

73

probably rather wide marginal basin now occur in the relatively narrow mountain

range and because deep water sediments in the Subbetic were uplifted to heights of

2000 m and more. This compressional component, however, did not at all obscure

the mentioned wrench characteristics and therefore must have been of far less

importance than the strike-slip component. Wrenching in the Subbetic was dextral. This follows from the inverse symmetry

and can also be deduced from structural data from the Crevillente Fault Zone, as

shown by Paquet (1972) and De Smet (1984). Prebetic structures in the Caravaca-Huescar area confirm that the Subbetic was

wrenched and that wrenching was of a dextral nature (Fig. 4). Fold axes in the

Prebetic have an average orientation of N40°E. They are en 6chelon arranged with regard to the northern front of the Subbetic and with regard to the axis of the

Crevillente Fault Zone (N65°E). Major faults in the area are also en 6chelon arranged. They have an orientation of N l l 0 o E and show dextral displacements.

Deformation in the Subbetic mainly took place in the Early Miocene (Garcia- Hernandez et al., 1980; Hermes, 1977, 1978a), but was not restricted to that period.

Folding and tilting occurred already in the Eocene and Oligocene (Hoedemaeker, 1973; Foucault, 1971; internal reports of the Amsterdam Universities). In parts of

the area deformation went on during the Middle Miocene (Geel, 1973) and the late Miocene (Hoedemaeker, 1973; Paquet, 1972). During Late Miocene times the

Prebetic area was also still affected (Foucault, 1971). Tectonic relations between the External Zone and the Internal Zone will not be

discussed in detail in this paper. In the Caravaca-Huescar area the contact between

the two is formed by the Velez Rubio Corridor (Geel, 1973; Roep, 1972), a narrow elongate depression, geologically characterized by intensive deformation and a

mixture of rock types. This extends eastwards into the "Zone Limite" of Paquet (1969). Deformation in the zone is generally ascribed to dextral transcurrent faulting. Although the fault zone was active during Early Miocene times (Hermes, 1978a; Durand Delga, 1980), contemporaneously with the process of wrenching in the Subbetic, the deformational pattern of the Subbetic does not seem to have been notably affected by this activity. Therefore movements along the Velez Rubio Corridor seem to have been subordinate to those in the Subbetic during that ' t ime. They may, however, have been of large scale importance in the preceding period

when the Internal Zone approached the External Zone.

THE EXTERNAL ZONE AS A WRENCH ZONE

On the scale of the entire External Zone similar arguments for wrenching can be put forward as in the Caravaca-Huescar area. Subbetic geology is in general characterized by intensive deformation, by a blocky structural style, varying structural directions and by stratigraphic anomalous contacts. The anomalous contacts sofar have been explained by most authors as the result of important overthrusting.

74

In the authors opinion, however, the outcrop pattern of rocks from the paleogeographic subzones (Fig. 3B) gives no evidence of large scale relative displacements in a direction perpendicular to the zonation. No tectonic units of the distal subzones

are found between those of the proximal ones or vice versa. It therefore seems likely

that the anomalies have to be explained by relative movements subparallel to the

zones. Such movements, of a dextral nature, are in fact suggested by the geometry of

the outcrop pattern. Rocks of the proximal subzones, probably up to the External Subbetic, can be traced to the islands of Ibiza and Mallorca (Azema et al., 1974;

Fourcade et al., 1982), which is about 400 km farther east than those of the most distal Subbetic ones. The latter on their turn can be traced to the area of the Arc of Gibraltar, about 250 km farther west than the outcrops of Prebetic subzones. This apparent relative displacement of subzones was already noted by Hermes (1978b),

who suggest that it is the result of dextral transcurrent movements between Prebetic

and Subbetic. A clear inverse symmetry with respect to a central fault zone, as was found in the

Caravaca-Huescar area, cannot be observed elsewhere in the External Zone. In the

western part of the zone, structures indeed show varying and even opposed direc-

tions of vergence, but, except for the southern border, northward vergences seem to dominate (cf. profiles by Hoeppener et al., 1963; Garcia-Hernandez et al., 1980).

East of the Caravaca-Huescar area only northward vergences occur. This is, however, in accordance with the wrench hypothesis, which predicts that the southern

part of the symmetric structure, i.e. the part south of the Crevillente Fault Zone, has moved away to the west. Only the northern part with its northward vergences was

left behind. Figure 10 illustrates the authors paleogeographic reconstruction of the External

PALEOGEOGRAPHIC L--~3~ RECONSTRUCTION .d~v,

OF THE ~ '~s~ ~ - i i~ EXTERNAL ZONE ,' , ~r~,. 7x~£ m~,¢,!

", }

N

" ~ , r ~ ' , ' ! ' \ v ~ , , ' , , ! r/Y "rl~ . . . . . . . ' '

Or ~ 100t 200, u m

Fig. 10. Paleogeographic reconstruction of the External Zone at about Oligocene times, just before the

wrench process started. See Fig. 3 for legend.

75

Zone. This reconstruction implies that the wrench process had a strike-sl ip/compression ratio of about 4 : 1 (400 km : 100 km). Garcia-Hernandez et al. (1980) give a paleogeographic reconstruction that is somewhat alike the one shown here, but which involves strike-slip motion of only about 250 km. These authors suggest that a phase of mainly strike-slip between the External and Internal Zone was followed by a phase of almost pure compression. The latter phase would have been responsi- ble for the origin of a nappe structure. In the present interpretation the deformation of the External Zone is considered as the result of more continuous oblique movements. After the Internal Zone had obliquely approached and nmde contact with the External Zone, causing deformations along their mutual boundary (the Velez Rubio Corridor), further movements were mainly accomodated within the External Zone and this resulted in a wrenched continental margin.

PLATE TECTONIC FRAMEWORK

Oblique plate movements, causing a dextral wrench process in the External Zone, seem quite possible from a plate tectonic view. The zone is situated along the boundary between the European and African plates, which can be traced from the Mid Atlantic ridge, along the East Azores Fracture Zone and the Horseshoe Seamounts to the Gibraltar area and the Mediterranean (Fig. 11). Near the Azores the boundary between the two plates forms a single lineament, but in an easterly

so°ao° ,¢ lo ° 21°°

25 24 21 21 12~ 25 • , ' ' . : ,. -.. .. . , -.

-. ".. • . : •

• . !

3 0 o

- .- " i . ~ . / ~ :: .: .i

: " : : ' ~ u ~ j ~ : : : i

-! / . ~ ... ... /.! ' : : ol 5 . / " ..

• ! , Z > / - ~ . ~, ;~:,

333~

\'\

• " i

° ~ E U R O

- ~ ~ ~AZORES 25 3i : .'

. . . . . EAST AZORES ~R~CT~ HORS(s~HOE ' SEAMOUNTS ~

v" .- / / •

' , , ' " ,I ° . ~ o ° ~ / A F R , C,~ ' " " 333, . • : C A , A ~ . . , S L

. f ' " . ; . /

' ' / i I ' , Y

Fig. 11. Plate tectonic position of the western Mediterranean area. Magnetic anomaly data are from Rona (1980).

76

direction the situation becomes more and more complicated. Large scale relative movements between the European and African plates were

reconstructed by Dewey et al. (1973). Biju-Duval et al. (t977) give detailed paleogeo-

graphic reconstruction maps of the Mediterranean area. Plate movements in the region of the Iberian plate, the Arc of Gibraltar and northwestern Africa are discussed in many papers (Andrieux et al., 1971, 1973; Arana and Vegas 1974;

Durand Delga, 1980). Paleogeographic reconstructions for this region, however, are

still facing major problems. Ages and amounts of displacements are only mentioned with caution.

Most authors suggest that from the Late Cretaceous onwards major right lateral strike-slip movements, followed by convergent movements occurred in the zone between the Spanish and the Moroccan Meseta. These movements must have mainly

taken place in areas directly north or south of the Arc of Gibraltar, since no major faults are running through the arc itself, as shown by Didon et al. (1973). For the area south of the arc Courbouleix et al. (1981) suggest big east west dextral fault

displacements during the Jurassic, but such deformations are not known from later periods. Concerning the area north of the arc dextral strike-slip faults are described by Didon (1973). Furthermore, as mentioned in the introduction, it is generally

assumed that the Late Mesozoic position of the Internal Zone /Alboran plate was farther east in the Mediterranean area. This position suggests that major movements

occurred along this northern arc side. Purdy (1975) reconstructed plate movements for the area of the Horseshoe

Seamounts. He points out that all motions between the African and European plates

during the last 72 Ma seem to have been accommodated along a single line of

weakness in the lithosphere, trending N-60°-E and running through southern Spain. He furthermore concludes that two major periods of relative motions occurred. A

slow compressive phase existed during the last 10 Ma, implying crustal shortening of an order of about 100 km perpendicular to the line mentioned. It was preceded at 60-72 Ma BP by right lateral strike-slip motion parallel to the line over a distance of about 600 kin. The given orientation and location of the line of weakness as well as the estimated amounts of compression and strike-slip are in agreement with the wrench hypothesis for the External Zone. The two phase theory and ages of these phases are, however, not in agreement, since deformation of the External Zone took

mainly place during the Early Miocene (15-22.5 Ma B.P.). The plate tectonic regime in the region of southern Spain after Middle Cretaceous

times is one of right lateral oblique displacements. The wrench system~ of the

External Zone reflects such a regime.

GENERAL CONCLUSION

Especially during the last years, large scale strike-slip phenomena are recognized in many orogens (Van de Fliert et al., 1980; Badham, 1982). Land geologists seem to

77

become generally aware of the possibility that orogenesis may be associated with

str ike-sl ip processes. In the case of the External Zone of the Betic Cordilleras

oblique convergent plate movements caused that the continental margin along Iberia

was wrenched. In particular, the rheologically weak deposits of slope and rise were

affected, while especially the intercalated rigid Jurassic layers controlled deforma-

tion. A very complex zone originated, with even locally a great variation of

structures and structural directions. Trends, reflecting the wrench process, first are

visible f rom the scale of the Caravaca -Huesca r area onwards.

Structural interpretation of the entire External Zone still faces many problems.

The classical nappe model has been tested for the last decades, but so far has not led

to a consistent explanation of the orogens main characteristics. In my opinion a

wrench concept, as discussed in this paper, offers more perspectives as a general

interpretational framework. Further testing the concept might at least be an eye

opener for the recognition of strike-slip phenomena and lead to a better understand- ing of this orogen.

ACKNOWLEDGEMENTS

The study in the Caravaca -Huesca r area formed part of the Betic Cordillera

Project carried out by the Amsterdam geological institutes. I wish to thank J.J.

Hermes and J.R. Van de Fliert, supervisors of the project, for the many fruitful

discussions at various stages of investigations and for reviewing the manuscript .

Al though we do not agree on all aspects of the wrench concept expressed above,

their ideas were of vital importance for its development. I thank H. Graven for his

share in the study of the Crevillente Fault Zone, Y.A. Baumfalk and J.E. Van Hinte

for discussions and mental support, and finally I thank the many students who provided field data and examined certain aspects of the wrench hypothesis.

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Documents

Wrenching in the external zone of the Betic Cordilleras, southern Spain